In cellular communications networks such as 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) networks, there are two types of deployments, namely a homogenous network and a heterogeneous network. A homogeneous network utilizes a single layer, or tier, of radio network nodes. In one particular example, all radio network nodes in a homogeneous network are high power nodes (HPN) such as wide area base stations serving macro cells. As another example, all radio network nodes in a homogeneous network are low power nodes (LPN), e.g., local area base stations serving pico cells. When there are similar load levels in the different cells of a homogeneous network, a wireless device, which is sometimes referred to as a User Equipment device (UE) or terminal, typically receives equally strong signals from a serving or measured cell and from a closest neighboring cell(s), especially when the UE is located in the in the cell border region. Therefore, in a homogeneous network, resource partitioning between serving and neighboring cells for the purpose of inter-cell interference mitigation is not as critical as in heterogeneous networks.
A heterogeneous network includes two or more layers of radio network nodes. In particular, each layer of the heterogeneous network is served by one type, or class, of base stations (BSs). Each layer, or set, of base stations has a fundamentally different set of attributes in one or more of the following: coverage extent or maximum transmit power (e.g., macro, micro, pico, or femto), carrier frequency (possibly multiple, overlapping, or non-overlapping with the carriers of other layers), and radio access technology (RAT). In one embodiment, a heterogeneous network includes a set of high power nodes (e.g., a set of high power or macro base stations) and a set of low power nodes (e.g. a set of low power or medium range, local area, or home base stations) in the same geographical region. A BS power class is defined in terms of maximum output power and other radio requirements (e.g. frequency error, etc.) which depend upon the maximum output power. The maximum output power, Pmax, of the base station is the mean power level per carrier measured at the antenna connector in specified reference condition. The rated output power, PRAT, of the base stations for different BS power classes is expressed in Table 1 below.
TABLE 1Base Station rated output power in LTE (FDD and TDD)BS classPRATWide Area BS- (note)Medium Range BS<+38 dBmLocal Area BS≦+24 dBmHome BS≦+20 dBm (for one transmit antenna port)≦+17 dBm (for two transmit antenna ports)≦+14 dBm (for four transmit antenna ports)<+11 dBm (for eight transmit antenna ports)NOTE:There is no upper limit for the rated output power of the Wide Area Base Station.As stated above, some of the requirements may also differ between BS classes. A wide area BS serves a macro cell, a medium range BS serves a micro cell, a local area BS serves a pico cell, and a home BS serves a femto cell. Typically, a wide area BS is regarded as HPN, whereas all the remaining classes of BSs can be regarded as LPN.
In a two layer macro-pico heterogeneous network, the macro cell and pico cell layers typically include wide area base stations, which are also known as macro base stations, and local area base stations, which are also known as pico base stations, respectively. The high data rate wireless devices located close to the pico base stations (i.e. in the pico layer) can be offloaded from the macro layer to the pico layer. A more complex heterogeneous deployment may include three layers, namely, a macro layer, a micro layer that is served by medium range base stations, and a pico layer. An even more complex heterogeneous deployment may include three layers, namely, a macro layer, a pico layer, and a home or femto layer.
With respect to a heterogeneous network, macro-cells are typically deployed to provide ubiquitous coverage while smaller cells are deployed to (a) boost overall capacity by serving hot-spots, or (b) address holes in the macro-cell coverage. In a heterogeneous network, there are many instances where a coverage area, or region, of one base station (e.g., a pico base station) is wholly contained within a coverage area of another individual, or set, of base stations (e.g., a macro base station).